1. Field Of The Invention
The present invention relates to bioprosthetic valves. More particularly, the present invention relates to a valve stent, including an upper ring formed of multi-segmented parabolic-shaped scalloped rings, the valve stent formed of a polymer and spring steel composite structure.
2. General Background
Valve prostheses have been utilized in surgical applications to replace various diseased or damaged natural valves, more particularly, heart valves including the aortic valve, the mitral valve and the tricuspid valve. In general, the failure of tissue heart valves have primarily been attributed to calcification and premature degeneration of tissue used to fabricate the valve. Although reasons for calcification of the tissue in these devices are still not well understood it is believed that excessive mechanical stresses are experienced by the tissue during their opening and closing function.
For example, in U.S. Pat. No. 4,343,048 issued to Ross, et al, entitled "Stent For A Cardiac Valve," the patented stent includes a metal base ring having metal legs projecting therefrom in a generally axial direction so that each of the legs can be resiliently deformed over substantially its whole axial length to take up strain in the valve without impairing its performance. The patent stresses that the base ring of the stent would be substantially rigid so that as pressure conditions occur within the heart, the base ring would not deform to any substantial extent. In addition, each leg would be shaped so that as it would project from the base ring, it would curve inwardly toward the axis of the stent in a "fishing rod structure" to avoid excessive stresses on the valve.
In recognizing problems in the present state of the art, an attempt to avoid valve failure has been the incorporation of a flexible polymer stent or valve frame. However, even with the use of the polymer stents, permanent deformation of these frames or stents due to polymer creep often results in failure of tissue valves using plastic frames.
As will be appreciated from the following description, it is possible in accordance with the present invention to provide a method to achieve complete flexibility of the valve frame without resulting polymer creep. A composite structure of polymer and spring steel is incorporated into the valve frame. Metal inserts of spring steel are injection molded into place at 120.degree. apart to form three vertical posts of the frame in the case of a tricuspid valve or 180.degree. apart to form two vertical posts in the case of a bicuspid valve. The flex life of this spring steel under worse conditions of physiological pressures would theoretically be infinite.
The frame would include an annular base ring defining an inflow orifice of the valve. If the valve is intended for use in the heart, this base ring is designed to proximate the physiological shape of the aortic valve annulus. If the valve is intended for use in a vein, the base ring is designed to proximate the physiological shape of the inner annular wall of the vein or vessel. In the preferred embodiment, there are molded into this ring three equally spaced frame posts joined by an upper ring comprising tri-segmented parabolic-shaped scallops. The upper scalloped ring defines generally three parabolic shaped one-third portions of the base between the center lines of the respective posts which together form a right cylinder of inside diameter of the valve. In the alternate embodiment, two oppositely spaced frame posts are molded into the base ring. The top of each frame post is joined by an upper ring comprising bi-segmented parabolic-shaped scallops. The parabolic shape of the scallops of the upper rings will reduce stress on the valve cusps. Cusp failure of existing tissue valves have been related generally to the ellipsoid shape of scalloping rings.
The scallops of the upper ring of the valve remain free and independent of the implant annulus to allow complete flexibility of the posts and the scallops. In the existing parabolic devices, the scallops form a part of the sewing ring which is tied down to the valve annulus during valve implantation thereby rendering the scalloping rings inflexible. This in turn would restrict the flex of the frame posts and expose valve cusps to higher stresses. In the present invention, the suture ring of the valve, which is constructed between the lower ring and the scallops of the upper ring, remains away and detached from cusp tissue.
The frame assembly may then be covered with fabric. Although this is not an essential feature, there are certain advantages to a fabric covered valve frame.